Electron emitting cathodes for irradiation machines



Oct. 13,1970 R. w. E. FULL ER 3,534,218

ELECTRON EMITTING CATHODES FOR IRRADIATION MACHINES Original Filed Sept.25, 1967 INVENTOkS ROBIN -w E. FULLER ATTORN EYS United States Patent3,534,218 ELECTRON EMITTING CATHODES FOR IRRADIATION MACHINES RobinWalker Ellecker Fuller, Wantage, England, as-

signor to United Kingdom Atomic Energy Authority, London, EnglandOriginal application Sept. 25, 1967, Ser. No. 670,263, now Patent No.3,479,551, dated Nov. 18, 1969. Divided and this application June 25,1969, Ser. No. 836,414 Claims priority, appIicationGSCir/e6a; Britain,Mar. 30, 1967, Int. Cl. H013 /50 US. Cl. 313-334 4 Claims ABSTRACT OFTHE DISCLOSURE An electron emitting cathode comprises a flexiblegraphite filament having a coating of a material which emits electronswhen heated. The filament may be tubular and loosely supported on ametal core wire.

This application is a division of application Ser. No. 670,263, filedSept. 25, 1967, now Pat. No. 3,479,551.

This invention relates to electron emitting cathodes for electron gunsand is particularly concerned with elongated emitting cathodes suitablefor use in electron irradiation machines.

In an electron irradiation machine intended for industrial processing,it is most desirable to use an electron gun with an elongated emittingcathode since it is then possible to dispense with magnetic scanning asa means of spreading the electron beam over the material beingprocessed.

Elongated emitting cathodes, generally referred to as emitters, have inthe past been constructed from refractory metals such as tungsten ortantalum. These metals can be used pure or in a treated state such asthoriated tungsten or tantalum coated with lanthanum boride to provide alow work function emitting film, but many practical difiiculties havearisen in attempting to provide a stable lanthanum boride coating onsuch materials.

Additionally, such metals are heavy and tend to be brittle after heatingin vacuum. In consequence, if emitting filaments more than a few incheslong are to be constructed, supporting and tensioning devices for thefilament are included every few inches. Various suspension systems havebeen suggested to overcome the problems but the brittle nature of a thinfilament, particularly when cold, remains as a limiting factor infilament life.

An object of the present invention is to provide a new and improved formof electron emitting cathode particularly suitable for use with electronirradiation machines.

According to the invention an electron emitting cathode comprises aflexible graphite or carbon filament having a coating of a materialwhich emits electrons when heated.

Preferably the filament comprises a plaited or woven graphite cord oflow mass/ unit length.

According to one feature of the invention the plaited or woven cord istubular and mountable on a wire such that an electron emitting cathodemay comprise a central core of refractory metal and a loose fittinggraphite or carbon sleeve surrounding said core.

Preferably the core is formed of tungsten or tantalum wire, and thesleeve is provided with a coating of a material which emits electronswhen heated.

The sleeve may be provided with a coating of lanthanum boride.

To enable the nature of the invention to be more readily understood oneembodiment of the invention will now be described, solely by way ofexample, with reference to the accompanying drawing which illustrates apreferred form of cathode construction.

The graphite or carbon sleeve materials are available commercially inthe form of tubular plaited cords of 1 mm. and 2 mm. outside diameters.The electrical resistance of the carbon cord is 1.5-2.5 ohms/cm. and forthe graphite 0.5-2 ohms/cm. Weights for 2 mm. diameter cords areapproximately 1.4 grams/metre.

In the preferred form of the invention as shown in the drawing thetubular sleeve 1 is loosely mounted on a core wire 2 and the ends 3 ofthe sleeve are copper plated to provide suitable end connections to thefilament electrical supply leads 4. The ends of the core wire are heldin conventional clamps 5 and connected via leads 6 to an electricalsupply to provide a core heating current. The core wire may be tensionedby using a simple spring mounting 7 or the core Wire and sleeve may besupported magnetically in a manner substantially as described in ourco-pending British patent application 35,474/ 66 which describes meansof providing a magnetic field transverse to the axis of an emittercathode such that the interaction between the field and the emittercathode, when carrying its operating current, supports the weight of theemitter cathode along its length.

One method of coating the graphite or carbon sleeve comprises applying aslurry, formed from lanthanum oxide, boron and carbon in water, to thecord, drying the cord in air, placing the cord in a vacuum vessel andoutgassing using external heaters and subsequently raising thetemperature of the sleeve to 1700 C. for a few seconds by passing anelectrical current through the wire core.

The slurry may be prepared by grinding together lanthanum oxide, boronand carbon in the proportions 9:3:1 by weight and adding distilledwater.

In one example the core wire and sleeve is supported in a filamentholder and brush coated with a prepared slurry. The sleeve is allowed todry in air and the assembly subsequently placed within a vacuumenclosure. The enclosure is pumped down to a hard vacuum, i.e. 10- torrapproximately, and the cord outgassed for several hours using anexternal heating source. Formation of an emitter coating is achieved bypassing an electrical current through the core wire to raise thetemperature to approximately 1700 C. for a few seconds, whereby a smallamount of lanthanum boride is formed on the surface layer of the sleeve.The sleeve is allowed to cool before application of a high tensionvoltage to the filament assembly and the filament is preferably operatedat a temperature 1000- 1100 C.

Other coating methods may include:

(a) painting the cord with a slurry of lanthanum boride in amylacetate,

(b) cataphoretic deposition of lanthanum boride using an electrolyte ofmethyl alcohol with a minor proportion of sulphuric acid or (c) paintingthe cord with a dilute organic cement to produce a tacky surface andsubsequently dusting with a dry lanthanum boride powder, allowing thecord to dry, and subsequently heat treating the coated cord at about1500" C. for 20 minutes in a vacuum approaching 10- torr.

It will be appreciated that the cord may be used without a central corewire, for example the coated cord may be formed as an elongated electronemitting cathode by taking a suitable length of cord and copper platingeach end to provide electrical connection points or alternatively thecord may be left unplated and the ends clamped between suitable softmetal members.

The cathode may be tensioned by using a simple spring mounting since theweight of a coated graphite cord 2 mm. dia. approaches 1 /2 ozs. per 100ft. length and the cathodes vary from a few inches in length to about 10feet. With a 4 ft. length of cathode the tension required to keep thecathode within acceptable limits of straightness does not exceed of thebreaking strength of the cord which is 10 lb. at room temperatures andincreases up to double this amount at elevated temperatures, say 1600 C.Advantageously, the coefficient of linear expansion of the coated cordis low being approximately 4X10 C.- and thus tensioning devices do nothave to counter instabilities due to excess changes in length. A cord 1mm. diameter and weighing approximately 1 oz./1OO ft. has also provedsuitable for some application.

I claim:

1. An electron emitting cathode comprising a central core of refractorymetal and a loose fitting carbonaceous sleeve surrounding said core.

2. An electron emitting cathode according to claim 1 wherein the core isformed by a tungsten wire and the sleeve comprises a plaited or Wovengraphite cord.

3. An electron emitting cathode according to claim 1 wherein the sleeveis coated with lanthanum boride.

4. An electron emitting cathode according to claim 3 wherein the ends ofthe sleeve are connected to the fila- Inent electrical supply and theends of the central core are connected to a secondary electrical supplyto provide a core heating current.

References Cited JOHN HUCKERT, Primary Examiner A. J. JAMES, AssistantExaminer US. Cl. X.R.

